1. Field of the Invention
The present invention relates to four-phase phase converters, and more particularly, to a four-phase phase converter used for QPSK modulation.
2. Description of the Related Art
FIG. 6 shows a conventional four-phase phase converter. In FIG. 6, a four-phase phase converter 100 includes a signal input terminal 101, a directive coupler 102 connected to the signal input terminal, unbalanced-to-balanced converters 103 and 104 connected to two outputs of the directive coupler 102, and signal output terminals 105, 106, 107, and 108 connected to two outputs of each of the unbalanced-to-balanced converters 103 and 104. The directive coupler 102, and the unbalanced-to-balanced converters 103 and 104 are formed of a combination of .lambda./4 microstriplines. Since their configurations are of general types, the descriptions thereof will be omitted.
In the four-phase phase converter 100 configured as described above, a signal input to the signal input terminal 101 is converted to two signals having phases 90 degrees apart in the directive coupler 102, and they are input to the unbalanced-to-balanced converters 103 and 104. Each of the signals input to the unbalanced-to-balanced converters 103 and 104 is converted to two signals having phases 180 degrees apart and output from the signal output terminals 105, 106, 107, and 108. As a result, one signal is divided into four signals having phases 90 degrees different from each other.
In the conventional case described above, however, one directive coupler and two unbalanced-to-balanced converters, namely, three phase shifters are required to obtain one four-phase phase converter. Since this requires an area for forming eight microstriplines and certain clearances between the phase shifters in order to avoid coupling between the phase shifters, the required area is large and therefore the cost increases. In addition, an assembling cost for assembling each phase shifter is also necessary. Furthermore, since the directive coupler and the unbalanced-to-balanced converters are manufactured independently, phase deviation caused in assembling becomes large.